Surface Gold Atoms Research Articles

Xylanase immobilized nanoporous gold as a highly active and stable biocatalyst

Dealloyed nanoporous gold, a nanostructured metallic sponge with tunable porosity and excellent biocompatibility, was employed as an effective support material for xylanase immobilization. Structure analyses revealed that the immobilization of xylanase was realized via chemical bonding of sulfur end-groups with gold surface atoms. The activity and stability for the immobilized enzyme were investigated under different conditions. And the immobilized enzymes were found to keep as high as 80% of the activity of free ones. Notably, these novel nanobiocatalysts showed quite impressive stability. Even after ten reaction cycles, this bio-nanocomposite could still retain ∼60% of the initial activity.

Effect of a thioalkane capping layer on the first hyperpolarizabilities of gold and silver nanoparticles

We have measured the first hyperpolarizabilities of thioalkane capped silver and gold metallic nanoparticles. The values found are βAgC 12−10 nm = (2.10 ± 0.23) × 10−26 esu for 10 nm diameter silver nanoparticles and βAuC 18−18 nm = (3.37 ± 0.08) × 10−26 esu for 18 nm diameter gold nanoparticles at the fundamental wavelength of 784 nm. By comparison to the corresponding values reported for citrate capped silver and gold metallic nanoparticles, after size corrections, decreases by factors of 4.3 and 6.5 respectively are observed. These decreases are tentatively attributed to the bonds formed between the gold and silver surface atoms and the sulfur atoms of the capping layer.

Conduction mechanisms in biphenyl dithiol single-molecule junctions

Based on density-functional theory calculations, we report a detailed study of the single-molecule charge-transport properties for a series of recently synthesized biphenyl-dithiol molecules [D. Vonlanthen et al., Angew. Chem., Int. Ed. 48, 8886 (2009); A. Mishchenko et al., Nano Lett. 10, 156 (2010)]. The torsion angle $\ensuremath{\phi}$ between the two phenyl rings, and hence the degree of $\ensuremath{\pi}$ conjugation, is controlled by alkyl chains and methyl side groups. We consider three different coordination geometries, namely, top-top, bridge-bridge, and hollow-hollow, with the terminal sulfur atoms bound to one, two, and three gold surface atoms, respectively. Our calculations show that different coordination geometries give rise to conductances that vary by one order of magnitude for the same molecule. Irrespective of the coordination geometries, the charge transport calculations predict a ${\mathrm{cos}}^{2}\ensuremath{\phi}$ dependence of the conductance, which is confirmed by our experimental measurements. We demonstrate that the calculated transmission through biphenyl dithiols is typically dominated by a single transmission eigenchannel formed from $\ensuremath{\pi}$ electrons. For perpendicular orientation of the rings a residual conductance arises from $\ensuremath{\sigma}$-$\ensuremath{\pi}$ couplings. But only for a single molecule with a completely broken conjugation we find a nearly perfect degeneracy of the $\ensuremath{\sigma}$-$\ensuremath{\pi}$ eigenchannels for the hollow-hollow-type contact in our theory.

Structure of the Au/Pd(100) Alloy Surface

The distribution of gold atoms on the surface of a Au/Pd(100) alloy with various gold coverages was explored using density functional theory (DFT) calculations and measurements of the low-energy electron diffraction (LEED) patterns. DFT calculation revealed the presence of first-, second-, and third-neighbor interactions. This contrasts the behavior of Au/Pd(111) alloys, where there were only nearest-neighbor interactions between the surface gold and palladium atoms. The presence of longer-range interactions was confirmed by LEED, which showed c(2 × 2) structures for palladium coverages between 0.5 and ∼0.75 monolayers (ML) and a (3 × 3) pattern between 0.5 and ∼0.85 ML. The surface structure was simulated using first-, second-, and third-neighbor interactions using Monte Carlo methods and was successfully able to reproduce the experimentally observed LEED patterns. The simulations were then used to calculate the variation in coverage of bridge-bonded carbon monoxide on the Au/Pd(100) alloy as a function of alloy composition, which also agreed well with experiment.

Mono- and multi-layer adsorption of an ionic liquid on Au(110)

Ultraviolet photoelectron spectroscopy (UPS), work function measurements, low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) have been used to study the adsorption and desorption of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide, [C(2)C(1)Im][Tf(2)N], on the (1×2) clean surface reconstruction of Au(110) in the temperature range 100-674 K. The ionic liquid adsorbed without decomposition, and desorbed without leaving any residue on the surface. For adsorption at room temperature a monolayer of strongly bound ionic liquid was formed with four interface states visible in UP spectra. STM at 100 K showed that the monolayer consisted of well-ordered rows of adsorbed ionic liquid aligned parallel to the close packed rows of surface gold atoms (the [110] direction) with a separation of ×2 (the same as the clean surface reconstruction) between the rows in the orthogonal [001] direction. Multilayer adsorption at room temperature occurred by droplet formation followed by smoothing of the droplets to a layered morphology with time. Heating caused multilayer desorption at temperatures in the 363-383 K range, followed by partial monolayer desorption at 548 K to produce a Au(110)-(1×3) reconstructed surface with sub-monolayer domains of ionic liquid. Desorption of the remaining ionic liquid at 600 K caused the gold surface to reconstruct back to the clean (1×2) reconstruction.

Density Functional Theory Study of the Adsorption of Nitrogen and Sulfur Atoms on Gold (111), (100), and (211) Surfaces

Nitrogen and sulfur atom adsorption on flat and stepped gold surfaces are examined by density functional theory. With detailed investigation of adatom location on the (111), (100), and (211) gold surfaces, nitrogen and sulfur atom adsorption are compared with reference to previous work on the oxygen/gold system. Sulfur adsorbed most strongly, followed by oxygen and nitrogen. The results demonstrate the preference for 3-fold over 2-fold and single-fold adatom coordination as well as the role of low coordinated gold surface atoms in increasing the adsorption energy for nitrogen, sulfur, and oxygen atoms. Pseudopotential curves, calculated adsorption energy as a function of surface position, and nudged-elastic band calculations explored adatom diffusion along the surface. The results indicated limited diffusion on the (111) and (211) surfaces. On the other hand, while nitrogen and sulfur atoms remained localized on the (100) surface, oxygen atoms showed facile diffusion. These results provide a reference for...

Chiral Au25 Nanospheres and Nanorods: Synthesis and Insight into the Origin of Chirality

Chirality in nanoparticles is an intriguing phenomenon. Herein, we have devised a well-defined gold nanoparticle system for investigating the origin of chirality in nanoparticles. We have designed chiral thiols (R- and S-isomers) and synthesized chiral gold nanoparticles composed of 25 gold atoms and 18 ligands, referred to as Au(25)(pet)(18), where pet represents chirally modified phenylethylthiolate -SCH(2)CH(CH(3))Ph at the 2-position. These optically active nanoparticles are close analogues of the optically nonactive phenylethylthioalte-capped Au(25)(pet)(18) nanoparticles, and the latter's crystal structure is known. On the basis of the atomic and electronic structures of these well-defined Au(25) nanoparticles, we have explicitly revealed that the ligands and surface gold atoms of Au(25)(pet)(18) play a critical role in effecting the circular dichroism responses from the nanoparticles. Similar effects are also observed in chiral Au(25) rods. The mixing of electronic states of ligands with those of surface gold atoms constitutes the fundamental origin of chirality in such nanoparticles.

Demonstration of a NOR logic gate using a single molecule and two surface gold atoms to encode the logical input

A logic gate has been implemented in a single trinaphthylene molecule. Each logical input controls the position of a surface Au atom that is brought closer or further away from the end of one of the naphthyl branch. Each Au atom carries 1 bit of information and is able to deform nonlocally and to shift in energy the molecular electronic states of the trinaphthylene. Probed at the end of the third naphthyl branch using scanning tunneling spectroscopy, the variations of the tunneling current intensity as a function of the Au atoms position measures the logical output of the gate. We demonstrate both theoretically and experimentally that these variations respect the truth table of a NOR logic gate.

Decoration with ceria nanoparticles activates inert gold island/film surfaces for the CO oxidation reaction

Ceria nanoparticles (<5 nm) were deposited on a polycrystalline gold film supported on SiO 2/Si substrate in an e-beam apparatus, and the catalytic activity of the inverse CeO 2/Au structure for the CO oxidation reaction was evaluated at ambient pressure in a recycle reactor. The gold film contained large crystal grains (lateral size in tens and hundreds of nanometers) and was nearly inert. CeO 2 nanoparticles deposited directly on the silicon substrate were also inert. However, decoration of the gold film with CeO 2 nanoparticles produced an activity enhancement of three orders of magnitude. This induced activation of the gold surface atoms at the interface with ceria nanoparticles was independent of the grain size of the gold film but proportional to the amount of CeO 2. No visible reconstruction of the Au surface upon ceria deposition was found by TEM. XPS analysis identified a high concentration of Ce 3+ in the ceria nanoparticles, but was not useful in identifying the state of the interfacial gold atoms. In stability studies, it was found that deactivation was accompanied by a decrease in the Ce 3+ concentration. The Au interfacial atom-TOF of the inverse CeO 2/Au film structures and Au/CeO 2 nanoparticles was similar, as was the apparent activation energy, E app = 49 ± 9 kJ/mol, indicating a common reaction mechanism.

Molecular Modeling Characterization of a Conformationally Constrained Monolayer-Protected Gold Cluster

We present a multilevel molecular modeling study aimed at elucidating physical and chemical properties of gold clusters capped by a monolayer of thiolated oligopeptides. The protecting peptides are based on the α-aminoisobutyric acid unit, form intramolecular C═O···H−N bonds, and can form intermolecular hydrogen bonds. This study is motivated by recent breakthroughs into the determination of crystal structures of small gold clusters protected with small thiolated molecules. Such structures are characterized by surface gold atoms in the so-called “staple motifs”, as opposed to the commonly assumed structures in which thiolates bind to a high-symmetry gold cluster. It is unclear, however, whether the staple motif is common to all kinds of protecting layers, especially those made of polypeptides that are largely stabilized by intermolecular hydrogen bonding. Structural and spectroscopic properties are presented to understand the nature of peptide−peptide interactions, their structural arrangements, and their...

Adsorption Behavior of Metal Cations on Gold Nanoparticle Surfaces Studied by Isothermal Titration Microcalorimetry

AbstractThe adsorption behavior of metal cations on gold nanoparticles was investigated by isothermal titration microcalorimetry (ITC) for the first time, and verified by transmission electron microscopy (TEM), and UV absorption spectroscopy. When transition metal cations were injected into the gold colloids, a binding isotherm with a sigmoidal response was obtained; and when instead of alkaline earth cations, the binding isotherm changed from the overall exothermic to endothermic nature. The binding enthalpy depends on the nature and concentration of added cations. The interactions of metal cations with the surface gold atoms and the COO− group of citrate salt that weakly bound on gold nanoparticles are the driving force for the binding; and the binding process is enthalpically driven. TEM images indicate that the addition of metal cations results in the growth and aggregation of gold nanoparticles. After the metal cations were added to the gold colloids, the intensity of the surface plasmon resonance adsorption band at 523 nm of gold nanoparticles decreased, and a new absorption at around 610 nm appeared. This is attributed to gold nanoclusters induced by the surface binding of the metal cations on gold nanoparticles, which is further confirmed by ITC results.

Weakly bound capping agents on gold nanoparticles in catalysis: Surface poison?

Abstract The influence of the concentration and nature of weakly bound capping agents on the catalytic activity and selectivity of gold nanoparticles is explored in the selective liquid phase oxidation of benzyl alcohol with oxygen and complemented with a detailed XPS analysis. Two nitrogen-based capping agents, dodecylamine (DDA) and poly(vinyl-pyrrolidone) (PVP) and different gold catalysts (supported and unsupported gold nanoparticles, a reference gold catalyst with and without exposure to capping agents) have been used. The colloid nanoparticle-based catalyst exhibits a high activity at mild conditions (TOF = ∼10 4 h −1 at 353 K in air). The interaction of alumina support–nanoparticle induces the presence of cationic gold and more surface gold atoms and, hence, catalytic activity although capping agent is still present. The activity is the highest for the largest gold particles (∼6.4 nm), correlating with sites in larger surface planes. Weakly bound capping agents lower the accessibility to the active sites and may act as catalyst surface poison. Selecting the appropriate capping agent for the nanoparticles synthesis (weakly adsorbing on the particle and promoting polycrystallinity), nanoparticles with twinning defects and enhanced catalyst activity are produced, compensating the negative effect of diffusional hindrance. On the other hand, the selectivity to benzaldehyde at higher conversions is dominated by gold particle size, increasing with decreasing size, and independent of the presence of capping agents and twinning defects.

Insights from Theory on the Relationship Between Surface Reactivity and Gold Atom Release

Density functional theory, informed by experimental studies, is used to investigate the interplay of surface morphology, the adsorption site of reactants, the nature of the interaction between adsorbates and the surface, the potential energy landscape for adsorbates on the surface, adsorbate coverage, temperature, and the dynamic evolution of these factors during adsorption and reaction. We summarize our current understanding of Au atom release on the (111) surface and the corresponding effects on adsorption and reactivity. Gold was selected for these investigations because of the recent intense interest in the activity of gold nanoparticles for several important catalytic reactions. Fundamental experimental studies on Au single-crystal surfaces have established that atomic O is extremely active for oxidation of CO and olefins, that the local bonding of O is an important factor in determining the reactivity and selectivity for oxidation, and that Au atom release is induced by electronegative adsorbates, such as O, Cl, and S. These experimental results guided our theoretical studies. Density functional theory is an extremely useful tool since it evaluates the energetics associated with the incorporation of gold into the adsorbate layer, while providing fundamental physical insight into the underlying cause of gold incorporation. We use our results from static DFT calculations along with ab initio molecular dynamics simulations to understand the effect of surface morphology on the activity of gold for CO oxidation. Our investigation of Au atom release and incorporation induced by electronegative atoms clearly illustrates the importance of using experiments in combination with theory to establish the importance of and the underlying reasons for metal atom release and the affect on bonding and reactivity.

Weakly bound capping agents on gold nanoparticles in catalysis: Surface poison?

The influence of the concentration and nature of weakly bound capping agents on the catalytic activity and selectivity of gold nanoparticles is explored in the selective liquid phase oxidation of benzyl alcohol with oxygen and complemented with a detailed XPS analysis. Two nitrogen-based capping agents, dodecylamine (DDA) and poly(vinyl-pyrrolidone) (PVP) and different gold catalysts (supported and unsupported gold nanoparticles, a reference gold catalyst with and without exposure to capping agents) have been used. The colloid nanoparticle-based catalyst exhibits a high activity at mild conditions (TOF = ∼10 4 h −1 at 353 K in air). The interaction of alumina support–nanoparticle induces the presence of cationic gold and more surface gold atoms and, hence, catalytic activity although capping agent is still present. The activity is the highest for the largest gold particles (∼6.4 nm), correlating with sites in larger surface planes. Weakly bound capping agents lower the accessibility to the active sites and may act as catalyst surface poison. Selecting the appropriate capping agent for the nanoparticles synthesis (weakly adsorbing on the particle and promoting polycrystallinity), nanoparticles with twinning defects and enhanced catalyst activity are produced, compensating the negative effect of diffusional hindrance. On the other hand, the selectivity to benzaldehyde at higher conversions is dominated by gold particle size, increasing with decreasing size, and independent of the presence of capping agents and twinning defects.

Oxygen activation on gold nanoparticles: separating the influence of particle size, particle shape and support interaction

The adsorption and dissociation of molecular O(2) on extended gold surfaces, isolated gold nanoparticles of different size and shape, and small gold clusters supported on TiO(2) have been investigated by means of DFT calculations. The influence of particle size and morphology on the mode of adsorption and activation of O(2) have been analyzed separately, and it has been found that O(2) dissociation is very sensitive to the arrangement of the gold surface atoms. The control of this variable through catalyst preparation opens one way to improve the activity of gold catalysts.

The role of gold adatoms in self‐assembled monolayers of thiol on Au(111)

AbstractThe role of Au adatoms in high‐density alkanethiol self‐assembled monolayers (SAMs) has been investigated. Surface reconstructions in ethylthiol (ET) SAMs due to gold adatoms have been studied using DFT calculations. Relative energies are compared based on binding and surface energies calculated for different lattices using the same unit cell. We have found a structure with two ET‐Au‐ET adatom moieties binding on top of surface gold atoms as the most stable structure. Our model is consistent with the structural parameters obtained from experiments. In particular, the simulated STM image displays the characteristic zig‐zag pattern. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Effects of chlorine and oxygen coverage on the structure of the Au(111) surface

We investigate the effects of Cl and O coverage on the atomic structure of the Au(111) surface using density functional theory calculations. We find that the release and incorporation of gold atoms in the adsorbate layer becomes energetically favorable only at high coverages of either O or Cl (>0.66 ML (monolayer) for O and >0.33 ML for Cl), whereas adsorption without the incorporation of gold is favorable at lower coverages. The bonding between the adsorbate and gold substrate changes significantly with coverage, becoming more covalent (less ionic) at higher Cl and O coverage. This is based on the fact that at higher coverages there is less ionic charge transfer to the adsorbate, while the electron density in the region between the adsorbate and a surface gold atom is increased. Our results illustrate that the O and Cl coverage on Au(111) can dramatically affect its structure and bonding, which are important features in any application of gold involving these adsorbates.

Theoretical Study of the Direct Synthesis of H2O2 on Pd and Pd/Au Surfaces

The direct synthesis of hydrogen peroxide on Pd and Pd/Au catalysts was investigated with first-principle DFT methods for periodic two-dimensional surfaces. A two-step reaction mechanism was proposed starting from a superoxo precursor state of the dioxygen molecule on Pd surface and its subsequent reaction with two hydrogen atoms situated over neighboring 3-fold positions. A competitive reaction of dioxygen dissociation leading to the nonselective formation of water was found. We have shown that the presence of surface gold atoms blocks this dissociation and increases the selectivity toward the main product, H2O2, which explains the experimentally reported data.

Kinetics of oxidative dissolution of gold nanoparticles in triton N-42 reversed micelles

The dissolution of gold nanoparticles as a result of a reaction with dispersed aqueous solution of Cl− and H2O2 solubilized in Triton N-42 reversed micelles (an oxyethylated surfactant) in n-decane was studied photocolorimetrically. An adequate description of the process kinetics is provided by the following autocatalytic scheme (in a Cl− + H2O2 excess): Au0 → Au+, Au0 + Aun+ → 2Au3+, n = 1 and 3. The reactions involve the formation and redox decomposition of intermediate complexes with the oxidizer on the surfaces of metallic particles. Dimensional factors associated with gold particles (the surface and reactivity of gold particles change during the process) and micelles (as nanoreactors) affect the process kinetics. The first effect is taken into account when data processing is performed in terms of the current number and effective charge density of surface gold atoms; therefore, it does not affect the observed rate constants kd1 and kd2. The second effect makes kd1 and kd2 dependent on the micelle size through changing the quality of the aqueous medium and the reactivity of the reaction components, mainly the activity of water and its hydration and adsorption abilities.

Surface plasmon resonance and magnetism of thiol-capped gold nanoparticles

Surface plasmon resonance measurements and magnetic characterization studies have beencarried out for two types of thiol-capped gold nanoparticles (NPs) with similar diametersbetween 2.0 and 2.5 nm and different organic molecules linked to the sulfur atom:dodecanethiol and tiopronin. In addition, Au NPs capped with tetraoctyl ammoniumbromide have also been included in the investigation since such capping molecules weaklyinteract with the gold surface atoms and, therefore, this system can be used as amodel for naked gold NPs; such particles presented a bimodal size distributionwith diameters around 1.5 and 5 nm. The plasmon resonance is non-existent fortiopronin-capped NPs, whereas a trace of such a feature is observed for NPs covered withdodecanethiol molecules and a bulk-like feature is measured for NPs capped withtetralkyl ammonium salts. These differences would indicate that the modification ofthe surface electronic structure of the Au NPs depends on the geometry andself-assembling capabilities of the capping molecules and on the electric charge transferredbetween Au and S atoms. Regarding the magnetization, dodecanethiol-cappedNPs have a ferromagnetic-like behaviour, while the NPs capped with tioproninexhibit a paramagnetic behaviour and tetralkyl ammonium-protected NPs arediamagnetic across the studied temperature range; straight chains with a well-definedsymmetry axis can induce orbital momentum on surface electrons close to the bindingatoms. The orbital momentum not only contributes to the magnetization butalso to the local anisotropy, giving rise to permanent magnetism. Due to thedomain structure of the adsorbed molecules, orbital momentum is not induced fortiopronin-capped NPs and the charge transfer only induces a paramagnetic spincomponent.